In a fuel injector, a solenoid control valve makes it possible to drive the movements of an injection needle via the variation of the pressure in a control chamber in which the head of the needle is located. When the solenoid valve is powered, it generates a magnetic field which attracts a magnetic core rigidly connected to a control valve slide, thus opening an evacuation channel for the fuel trapped in the control chamber, and, at the end of powering, a compression spring housed in a bore provided in the middle of the solenoid valve pushes back the magnetic core, and the control valve slide closing said evacuation channel again.
The flow of fuel injected each time the solenoid valve is powered is a functional characteristic which is dependent, among other parameters, on the compression of the spring and on the force that it exerts on the magnetic core. Yet, once the injector has been assembled, the compression of the spring is the resultant of dimensions, none of which can be changed: spring length, length of the bore, position of the magnetic core relative to the solenoid valve, etc. The result is a dispersion of said flow characteristic which covers a wide tolerance range which can only be restricted by tightening the manufacturing tolerances of the individual dimensions. However, this possibility has these technical limits, and the width of the current tolerance range is at the expense of improving the production quality of the injectors.